1. Field of the Invention
The present invention relates to a developing device in which toner particles are supplied to a static latent image formed on a peripheral surface of a photoconductive drum in an electrophotographic manner, and an image forming apparatus provided with the developing device.
2. Description of the Related Art
A predetermined developing device is used in an image forming apparatus which is operable to perform an electrophotographic manner. In the developing device, toner particles as a developer are supplied to a static latent image which is formed on a peripheral surface of a photoconductive drum in accordance with an image data. A toner image formed on the peripheral surface of the photoconductive drum by supplied toner as mentioned above is transferred onto a sheet which is an image bearing material conveyed from a sheet storage section with rotation of the photoconductive drum about an axis. A fixing device is provided on an immediately downstream of the photoconductive drum to perform a fixing process onto the sheet. A fixing process is conducted on the sheet onto which the toner image is transferred to fix the toner image on the sheet by heat. Then, the sheet is discharged to an outside after the completion of the fixing process.
The developing device includes stirring means for stirring a developer which includes only toner particles in the case of the one-component system, or a mixture of toner particles and carrier particles in the case of the two-component system, a cylindrical developing sleeve for supplying developer particles being stirred by the stirring means to the peripheral surface of the photoconductive drum, and a housing accommodating these members. In the case of the one-component system, toner particles supplied from a predetermined toner cartridge into the housing are stirred without adding an agent. In the case of the two-component system, toner particles are mixed and stirred with carrier particles put by a predetermined amount. Then, toner particles are supplied onto the peripheral surface of the photoconductive drum uniformly by a peripheral surface of the developing sleeve rotating about an axis. A blade is provided closely above the peripheral surface of the developing sleeve arranged along the peripheral surface of the photoconductive drum at a predetermined space. The blade removes surplus toner particles from the peripheral surface of the developing sleeve to thereby prevent excessive supply of toner particles to the photoconductive drum.
Meanwhile, there is the problem that when toner particles conveyed with the rotation of the developing sleeve reach the blade and some of the toner particles stray from a middle portion of the blade to the both ends of the blade, and disadvantageously leak from the respective ends of the developing sleeve.
In order to solve this disadvantage, Japanese Unexamined Patent Publication No. HEI 2-262171 discloses that a magnetic member is provided so as to face each of opposite end portions of a peripheral surface of the developing sleeve at a spacing from the peripheral surface of the developing sleeve so that the magnetic members and a magnet included in the developing sleeve form a magnetic brush having magnetic lines of force gathered on the peripheral surface of the developing sleeve. Accordingly, by taking such measure, the magnetic brush functions as a sealing member for a gap between the magnetic member and the developing sleeve. Consequently, leakage of toner particles is prevented.
Meanwhile, in the case where the structure for preventing leaking of toner particles disclosed in the Japanese Unexamined Patent Publication No. HEI 2-262171 is adapted, since a gap between an arc-shaped inner surface of the magnetic member and the peripheral surface of the developing sleeve is constant in a peripheral direction, a toner-restricting force of the magnetic brush is constant from upstream end to downstream end of the magnetic member. Thus, there exists an advantage that the toner particles are uniformly prevented from leaking in the area covered by the magnetic members. On the other hand, if the toner particles which are to be conveyed by the developing sleeve while restricted by the magnetic brush are deviated from the downstream end of the magnetic member, toner particles are released from the quite strong restricting force of the magnetic brush. Consequently, toner particles are scattered around by a reaction to cause a disadvantageous leakage of toner particles.
In the case where the gap between the downstream end of the magnetic member and the peripheral surface of the developing sleeve is narrowed by an accidental error in mounting the magnetic member, the toner particles moved to the downstream while restricted by a magnetic force is compressed in the narrowed gap. Accordingly, at the downstream end of the magnetic member, the toner particles which lose a space to move are forced to move to an inner portion of a seal (magnetic brush portion) having a space. Consequently, the magnetic restricting force with respect to toner particles is substantially lowered, and toner particles are caused to scatter around.